In recent years, in work machines such as hydraulic excavators, there is known a hydraulic circuit, a so-called closed circuit, in which connections in a closed circuit fashion are made to feed hydraulic oil from a hydraulic pump being a pressure generating source directly to a single rod hydraulic cylinder being a hydraulic actuator and in which the hydraulic oil after used in driving the single rod hydraulic cylinder to perform a given work is returned directly to the single rod hydraulic cylinder. On the other hand, as opposed to the closed circuit, there is also known a hydraulic circuit, a so-called open circuit, in which hydraulic oil is fed from a hydraulic pump to a single rod hydraulic cylinder through a throttle configured by a control valve and in which the return hydraulic oil from the single rod hydraulic cylinder is drained into a tank. Compared with the hydraulic circuit of the open circuit type, the hydraulic circuit of the closed circuit type is advantageous in fuel consumption performance because a pressure loss caused by a throttle is little and because regeneration by the hydraulic pump is possible with the energy that the return hydraulic oil from the single rod hydraulic cylinder possesses.
Further, Patent Literature 1 discloses prior art in which closed circuits of this kind are combined. In Patent Literature 1, there is installed a first closed circuit in which a hydraulic pump being an oil pump for operating a boom cylinder being a single rod hydraulic cylinder is connected to the boom cylinder in a closed circuit fashion, and there is also installed a second closed circuit in which a hydraulic pump for operating an arm cylinder being a single rod hydraulic cylinder is connected to the arm cylinder in closed circuit fashion. Furthermore, an open circuit is installed in which a hydraulic pump for operating a bucket cylinder being a single rod hydraulic cylinder is connected to the bucket cylinder through a control valve, and a distribution circuit that distributes the hydraulic oil discharged from the hydraulic pump of the open circuit to the boom cylinder and the arm cylinder is provided to branch from a side closer to the hydraulic pump than the control valve in the open circuit.